'It was always my intention eventually to return home to the United States, but I'm going years early because in all conscience I can no longer participate in a corrupt and dysfunctional system that is dishonest in its treatment and management of children with special needs.'

Janis Newcomen throws her hands in the air in exasperation. She is part of a system that is supposed to provide for the needs of Britain's most vulnerable children, those with conditions such as autism, Attention Deficit Hyperactivity Disorder and severe learning difficulties. But she has had enough.

In seven years as an NHS neuropsychologist, Newcomen has been so disgusted and upset by what she has seen that she is packing her bags and walking away. She says she can no longer bear to watch children and their families let down again and again. As a specialist who is supposed to provide help she says that she feels 'handcuffed', forced to accept hidden waiting lists, discrimination and constant cost cutting. She says she is officially prevented from making recommendations that could safeguard children's futures.

'The system is in crisis,' she said. 'But it is like the emperor's new clothes - nobody is willing to tell the truth.'

Recent research has revealed that autism and associated behaviour disorders among children are far more prevalent than was previously thought. The debate over whether such pupils should be taught in mainstream or special schools has also been reignited by reports that the present government policy, which leans towards inclusion, is not working properly.

Researchers who studied 56,946 children in south London found that almost 0.4 per cent had 'classic' childhood autism and just below 1.2 per cent had autism spectrum disorders, including Asperger's syndrome and milder forms.

Until the Nineties, the generally accepted figure in Britain was four to five cases of autism per 10,000 people - 10 times lower than the rate suggested in the new study.

It was an Observer investigation last May that triggered Newcomen's decision to speak out. She found the story of parents struggling to get their children's needs assessed and met by local education authorities and schools depressingly familiar. Now, as she prepares to leave her job at the Sussex Partnership NHS Trust, she no longer fears the consequences.

Newcomen talked from her office in Hastings, East Sussex, where she carries out specialist assessments and diagnostic work mainly with autistic children. Parents of such pupils experience an ordeal: 'If you have a child [with special educational needs], it is like Russian roulette. If you are passive, you could be waiting years, but if you have a loud voice you will be seen.'

It is wrong, she argues, that so many of the children she sees have been excluded from school because of behaviour directly related to their condition, which they have no control over. 'There are federal laws against that in the US - if it is not discrimination, I don't know what it is.'

Moreover, it will cause serious damage in the long run as children are brought up to feel like failures, she argues. 'We are excluding these children from primary school. What sort of message does that give? We are excluding them from school, from society, from having any existence.

'I think we [health practitioners] are handcuffed: we see the problems but are not able to help.'

Newcomen, 55, believes the school environment has a huge impact on vulnerable children's lives. Many autistic children she sees are being harmed because they are taught in large classes in busy, mainstream schools, she said. Yet there is nothing she can do: she is not allowed to recommend that they be moved, even if she thinks it could transform their behaviour and make life far easier for their parents.

'All we can do is provide information. We cannot say the child would benefit from a special school. We cannot make recommendations.' As an NHS worker, she said, any suggestions about where a child should be schooled would be met with anger from the local education authority.

There has long been a national debate about inclusion - whether to teach children in mainstream or special schools. Proponents say that with enough adjustments any child can be accommodated in the mainstream. Others say there is too little money to make this a practical reality for all children, and some pupils - particularly those with severe behavioural difficulties - benefit from smaller class sizes and more intense support in a special school.

While the government denies it has a policy of inclusion, a damning House of Commons select committee report published this month disagreed. The MPs said the present provision was not 'fit for purpose' and that inclusion was the message the government was sending to the local authorities.

It is certainly the message that has reached Newcomen and her colleagues. 'We are told the goal is inclusion and the goal is not to statement [issue an analysis of the child's needs with costs attached], because that needs extra money.'

The results are devastating, she argued. 'It is perfectly possible for an autistic child to be happy and well adjusted. They do not have to be violent and unhappy.' She believes inclusion could work well if every school was properly resourced, but under current conditions three out of four of the autistic children she sees would benefit from a place in a special school.

She believes that health workers are stymied because cost, rather than clinical need, drives everything. 'You are trying to work with families with incredible distress, and no one seems to care as long as the boxes are ticked.'

One tick-box is to see children for a one-off appointment quickly so the figures on how long they have to wait stay low. According to Newcomen, paediatricians are expected to make a diagnosis and then send the family away with a brochure and a telephone number for the National Autistic Society. They are often not given repeat sessions, even when the doctors think these are needed. Those who are given more sessions are placed on a second, much longer, waiting list which, she says, is 'kept secret'.

Ideally, families would get quarterly therapy sessions on how to understand and cope with their child's behaviour, said Newcomen, but they are not getting that support. She helps by running her own independent clinic offering it to families on a one-off basis. 'I go home at night to peace and quiet,' she said. 'These parents never have any downtime and it can have a destructive impact on siblings.'

It is the endless list of patients who have been failed by the system that angers her most. One mother she has worked with has a son who is autistic and can be dangerous. 'The mother has warned the school, but a statement has been turned down again and again. We are not allowed to say he needs a statement. Instead a mysterious panel has decided to reject that child, and he has no place in a special school.'

But Newcomen points out that because the boy's autism means he can be extremely violent he is not able to go to his school full time. 'How can his needs be being met in mainstream if he can't go in for a full day? His mother is now out of work [so that she can look after him], so they have lost money. He is being discriminated against. He is not being educated because of his disability. It is heartbreaking.'

At least he got a diagnosis early. Newcomen regularly sees patients in their mid-teens who are profoundly disabled but have gone through the system with no one ever noticing it. A 15-year-old girl referred to her had been running away, stealing and taking drugs. Newcomen assessed her as having severe learning disabilities. 'Her cognitive ability was extremely low - 99.9 per cent of the population score higher. That put a completely different slant on her behaviour.'

In fact, failure to help these children will lead to massive social problems, according to Newcomen. Without the right support, many could go on to behave antisocially and aggressively. The choice, she says, is: 'Invest now or build prisons later.'

Yet investing is the last thing NHS managers want to do, in her eyes. It took her months to get approval to buy a £135 diagnostic testing kit. She had to make presentations to a committee and a 'user group'. 'Asking for new equipment is agony and it made me think twice about ever asking again. Everyone is stretched like a rubber band and it is going to snap.'

Newcomen is clear that the fault does not lie with clinicians or teachers who she 'has nothing but admiration for'. But everyone is struggling: 'I feel awful. I can't provide the support. We diagnose and send them to an environment that will harm their needs.' It is the parents who get the fallout, she adds.

'I am frustrated. There is a discrepancy between what you hear the government say - that everything is wonderful - and what you see.' It is this daily reality that has pushed Newcomen to leave the UK.

She says she is not alone. She hears 'moaning, anger and frustration' from colleagues who also talk about resigning from their jobs. For her, there is nothing more she can do: 'I am feeling low on energy, very drained and hopeless.' Working in the NHS, trying to help the children who most need it, has become 'soul-destroying', she said.

That is why, on 27 August, she will travel with her husband to Southampton and board the Queen Mary 2 to sail back to Maryland.